omxil: Add reference counting around the OMX Core loading and initialization

omxil: Add reference counting around the OMX Core loading and initialization

This avoids deinitializing the global OMX Core if a second
instance of the module is initialized (e.g. for testing whether
it can handle decoding another stream) and deinitialized while
the first instance still is active.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>